For internal combustion engines, such as diesel engines, nitrogen oxide (NOx) compounds may be emitted in the exhaust. To reduce NOx emissions, a selective catalytic reduction (SCR) process may be implemented to convert the NOx compounds into more neutral compounds, such as diatomic nitrogen, water, or carbon dioxide, with the aid of a catalyst and a reductant. The catalyst may be included in a catalyst chamber of an exhaust system, such as that of a vehicle or power generation unit. A reductant, such as anhydrous ammonia, aqueous ammonia, or urea, may be introduced into the exhaust gas flow prior to the catalyst chamber. In conventional implementations, ammonia is produced from reductant solutions stored on board of the vehicle. To introduce the reductant into the exhaust gas flow for the SCR process, an SCR system may dose or otherwise introduce the reductant through a dosing module (doser) that vaporizes or sprays the reductant into an exhaust pipe of the exhaust system upstream of the catalyst chamber. Conventional reductant dosing processes may suffer from aftertreatment system corrosion, solution condensation at low temperatures (which hinders catalyst performance), a need for a reductant tank that must be refilled when emptied, and precise dosing control via, for example, spray nozzles which are complex and expensive to manufacture.
The ammonia synthesis reaction uses nitrogen and hydrogen to make ammonia (3H2+N22NH3), typically at high pressures (50-200 atmospheres). Conventionally, systems for providing on-board ammonia synthesis include H2/N2 tanks or fuel reforming as the source of H2. Furthermore a high pressure ammonia synthesis loop is often used, similarly to that which is used in industrial processes. The complexity of conventional on-board ammonia synthesis systems requires significant effort and space to execute and is the largest limitation hindering commercial adoption.
A water-gas shift (WGS) catalyst uses water and carbon monoxide to produce hydrogen and carbon dioxide (H2O+COCO2+H2). WGS catalysts come in many varieties, depending upon their application conditions. Low- and high-temperature shift (LTS and HTS) catalysts are the most commonly used varieties and are typically operated at temperatures of 180-275° C. (LTS) and 300-450° C. (HTS). Other varieties include a medium-temperature shift (MTS) WGS catalyst for a temperature range of 190-330° C. and a sour-gas shift catalyst for use with sulfur-containing gases. WGS catalysts can also perform across a wide pressure range as low as 1/10th of an atmosphere to 30 atmospheres of pressure.